City of Sin

When contract killer Shadow is sent to Vegas to kill Selene Van'Doren he figures she'll be an easy mark. That is, until the moment his finger is on the trigger and he finds himself staring into the eyes of a woman he once loved desperately who was supposed to have died years ago.
Selene survived the worst kind of hell and has risen to become the most powerful woman in the Las Vegas crime world, she thought she'd made her peace with her demons until the man she'd loved and lost, arrives in Vegas to kill her.
Forced to confront a past long dead and buried, and with Selene's enemies closing in, Shadow begins to realize that when you play by Vegas rules, you win or die...